This study is designed to investigate brain mechanisms underlying memory processes. We propose to use PET to measure brain metabolism at rest and during the performance of carefully-designed memory tests. Comparison of brain metabolism during the different test conditions will provide information about the brain areas that are involved in mediating specific aspects of memory. Aged subjects and patients with Alzheimer's disease will also be tested, to understand the memory changes in these individuals. Three groups of subjects will be studied: young (age 20-40) healthy individuals; a group of healthy elderly individuals, and subjects with probable Alzheimer's disease. For the initial activation task, we will use a computerized continuous recognition memory task. Comparing the pattern of energy metabolism in the brain of a subject at rest to that seen while he is performing a task allows us to begin to isolate the areas of the brain associated with task performance.